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Frank Marsh, 80, one of the nine patients taking part in the trial at Glasgow's Southern General Hospital, told the BBC he had seen improvements in the use of his left hand.

"I can grip certain things that I never gripped before, like the hand rail at the baths, with my left hand as well as my right," he said.

"It still feels fairly weak and it's still a wee bit difficult to co-ordinate but it's much better than it was."

He added: "I'd like to get back to playing my piano."

His wife Claire said: "He had reached a plateau and wasn't really improving [after his stroke]. But following the operation he is able to do things he couldn't do before, such as make coffee, dressing, and holding on to things."

The study involved patients who suffered strokes some time ago and had shown no signs of making any further spontaneous improvement.

Prof Muir said the results were "at the present time not what we would have expected in this group but far from being able to say whether it's something specifically related to the cells".

He told BBC Radio 4's Today programme: "We know that some of the cells will survive and potentially turn into relevant tissue. We also suspect that a large part of what we do is kick-starting repair processes that are already present in the body.

"So there's probably a mixture of things going on. Quite what it is that's happening in the patients, we won't know for some time to come."

The stem cells were created ten years ago from one sample of nerve tissue taken from a foetus.

Scientists said the PISCES study is the world's first clinical trial examining the safety of neural stem cell treatment in stroke patients.

The first phase of the research focused on whether there were any safety concerns about the treatment.

None of the patients treated had experienced any adverse effects, Prof Muir said.

The initial results will be presented at the European Stroke Conference in London today, with full results due to be published next year.

Plans for a second phase, which will further explore how effective the treatment is, are due to be submitted to UK regulators in early July.

Around 20 patients will take part if it is approved, all of whom will have suffered a stroke within a few weeks.